[Haskell-cafe] a simpler way to declare typeclass instances

[MigMit]

Well, in this case it would look like a definition of a completely independent function (=), which, obviously, clashes with the already defined one.

In general, it's a good idea to keep things that don't make sense without some previous declaration inside the block, established by that declaration.

> On 25 Oct 2014, at 19:42, Alexey Muranov <alexey.muranov at gmail.com> wrote:
> 
> Hello,
> 
> i am trying to understand how typeclasses work.  I know that they can be used as follows (from the manual):
> 
>    data Foo = Foo {x :: Integer, str :: String}
> 
>    instance Eq Foo where
>       (Foo x1 str1) == (Foo x2 str2) = (x1 == x2) && (str1 == str2)
> 
> I am wondering, why is the following seemingly unambiguous syntax not allowed too?
> 
>    data Foo = Foo { x :: Integer, str :: String }
> 
>    instance Eq Foo
> 
>    (Foo x1 str1) == (Foo x2 str2) = (x1 == x2) && (str1 == str2)
> 
> If it was allowed, it seems that it could also be applied to records:
> 
>    class HasName r where
>        name :: r -> String
> 
>    data Bird   = Bird { name :: String, wingNumber :: Integer }
>    data Person = Person { name :: String, likesBirds :: Bool }
> 
>    instance HasName Bird
>    instance HasName Person
> 
> Alexey.
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe